An analog to digital converter is a device which expresses the ratio of an analog input voltage to a reference voltage as a digital number. There are many known principles of analog to digital conversion as discussed, for example, in an Electronic Design Practical Guide to Analog to Digital Conversion, parts 1, 2 and 3 by H. Schmid in ELECTRONIC DESIGN, Dec. 5 and 19, 1968 and Jan. 4, 1969. The accuracy and resolution of these converters and systems in general are typically adversely affected by signals experiencing a DC shift when passing through a chain of circuit blocks.
The accuracy and resolution of converters are typically adversely affected by deviations of resistance ratios. Problems associated with converters having operational amplifiers include open loop gain deviation from prescribed values, input voltage and input current offsets, and finite input impedance. The speed of conversion is also limited by the slew rate of operational amplifiers. When using a transformer in the implementation of a converter, difficulties in preventing a D.C. voltage component from occurring across the transformer primary and eliminating offset and gain errors in the rest of the circuitry are experienced. Measurement of D.C. or slowly varying analog signals are impractical with prior transformer implementation.
Gain stabilization for a cascade composed of serially connected circuit blocks is generally achieved by ensuring a specified gain for each separate block. Local negative feedback with high loop gain in each block requires frequency compensation to ensure stability. This frequency compensation limits a converter's speed of conversion.